Switches of the kind mentioned above are designed in the form of circuit breakers and have a rated current carrying capacity of at least several kA in the voltage range of more than several kV. In general, these switches are used as generator switches and therefore contain a contact arrangement which can be loaded with high rated currents and has two switching pieces—in each case containing a rated current contact and an arcing contact. A current path containing the rated current contacts is used to carry the predominant part of the operating current, whereas a power path, which is connected in parallel with the rated current path and contains the arcing contacts, only carries the whole current for a short time when interrupting a generally extremely powerful current. The arcing contacts then draw a powerful switching arc, which causes a greater or lesser degree of contact wear depending on the amount of work performed by it. It is important for the switching behavior of these switches that, when switching off, the arcing contacts separate within a certain time interval after the rated current contacts and, when switching on, close within this time interval before the rated current contacts. However, the length of the arcing contacts along an axis, along which the two switching pieces move relative to one another during a switching operation, changes as a result of the contact wear. In the case of severe contact wear, the time can be less than this time interval and accordingly the switching capability of the switch can be greatly impaired.
A switch of the kind mentioned in the introduction is disclosed in DE U 20 2007 018 709 U1. With this switch, the contact wear of the arcing contacts is determined with the help of sensors and a signal measuring unit while the switch is operating, that is to say when the contact arrangement is closed, and the user of the switch is informed of the current condition of the arcing contacts caused by the contact wear by an indicator which is connected to the signal measuring unit. If the time is less than a time interval which corresponds to a critical value of the reduced length of the arcing contacts due to wear, the user is informed by means of a visual, in particular colored, and/or an audible signal that the remaining life of the switch is only relatively short.
Vacuum switches which in each case have a contact arrangement located in an evacuated vessel are disclosed in U.S. Pat. No. 6,002,560 A and U.S. Pat. No. 7,098,418 B1. The contact arrangement has two contacts which butt against one another head-to-head when the contact arrangement is closed. A moving contact of the two contacts is rigidly connected to a rod, which is arranged outside the vessel, of a transmission mechanism which transmits drive force. With these switches, while the switch is in operation, the contact wear is indicated in each case with the help of an indicator which is fixed directly to the rod or to an arm of a two-arm lever of the transmission mechanism which is hinged to the drive bar. Since the two contacts butt against one another head-to-head at the points which are at risk of wear, the indicator shows the sum of the length change of the two contacts and therefore the remaining life of the switch as a function of the position of the transmission mechanism which transmits drive force to the contact arrangement.
In US20080217297 A, in order to determine the contact wear for a circuit breaker having a rated current contact and wearing contact system, it is proposed to measure the concentration of a volatile marker substance. With this switch, the marker substance is contained in the stationary wearing contact pin and, on release, can be detected by a gas chromatograph. The concentration of the marker substance is said to behave proportionally to the wear on the contacts.
A circuit breaker having a single contact arrangement and a device for detecting the wear of the contacts is disclosed in FR2346837. The contact arrangement has two contacts which butt against one another head-to-head when the contact arrangement is closed. One of the two contact pieces is movable and is driven by the switch drive via levers. The switch drive also moves a lever which is connected directly to the drive and is located outside the housing and which reproduces the angle of rotation of the drive axis and therefore indirectly indicates the position of the moving contact piece outside the housing. This makes a detection of the wear of the contacts, which is mechanically coupled to the switch drive and is therefore dependent on the functional condition of the drive, possible.